It is known in the art of loudspeaker design to provide a speaker motor case to enclose a voice coil actuator for a loudspeaker. The voice coil is typically carried by a coil carrier. A small signal audio input is applied through an audio amplifier and a drive amplifier to produce a driving current for the loudspeaker. The input drive current is typically in a sinusoidal waveform. The driving current is applied to the voice coil, and pursuant to Lorenz's law, actuates the voice coil, causing voice coil excursion, or movement of the voice coil and carrier in an upward and downward axial direction.
One problem with the actuation of the voice coil arises when the input driving current exceeds a desired level. At this point, the amplified sinusoidal waveform, which serves as the driving current, is clipped at its peak amplitudes. The clipping in the input signal causes distortion in the output of the loudspeaker.
When the input driving current exceeds its desired level, the voice coil actuator also causes the coil carrier to move beyond its upper and lower position limits in the speaker motor case. The excursion of the voice coil carrier beyond these position limits in the speaker motor case can cause physical damage to or destruction of the voice coil.
Therefore, it is desirable to provide a device that actively controls the amplitude gain of the driving current input to the actuator. A need also exists for a device that actively prevents physical damage to the voice coil by controlling the amplitude of the voice coil excursion when damage to the voice coil is imminent. It is also desirable to provide a device that physically prevents the over-actuation and resultant physical damage of the voice coil.